Film with controlled gas permeability

ABSTRACT

An extrudable vinylidene chloride composition is useful in packaging films where controlled CO2 and O2 transmission rates benefit the packaging of products such as gassing cheese. The composition, and film made therefrom, comprises 100 parts by weight of at least one vinylidene chloride copolymer; between 4 and 15 parts by weight of a plasticizer; and between 4 and 15 parts by weight of an acrylate/styrene copolymer; wherein the total amount of plasticizer and acrylate/styrene copolymer comprises at least 9 parts by weight.

This application is a continuation of U.S. patent application Ser. No.08/418,743, filed Apr. 7, 1995, and now issued as U.S. Pat. No.5,726,229, which was a divisional of U.S. patent application Ser. No.08/341,819, filed Nov. 18, 1994, and now issued as U.S. Pat. No.5,538,770.

BACKGROUND OF THE INVENTION

The present invention relates to films made from vinylidene chloridepolymers or copolymers; more particularly to a method, composition, andfilm for controlling the oxygen and carbon dioxide transmission andstickiness, and enhancing the free flowability and thermal stability ofmonolayer and multilayer packaging films having a plasticized layer ofvinylidene chloride copolymer film.

Thermoplastic packaging films made of vinylidene chloride copolymer,here referred to as "saran", have long been used to package food such ascheese, fresh meat, etc. Saran is a good barrier to the transmission ofoxygen.

Methods of producing a multilayer film having a layer of saran aredisclosed in U.S. Pat. No. 4,112,181 (Baird) and U.S. Pat. No. 3,741,253(Brax et al), both incorporated herein by reference. Saran is typicallyplasticized and stabilized for purposes of extrusion. One example isepichlorohydrin/bisphenol A, and epoxy resin, and 2-ethyl hexyl diphenylphosphate.

U.S. Pat. No. 5,202,188 discloses a vinylidene chloride compositioncomprising 1 to 4% plasticizer, 1 to 4% acrylate/styrene polymer, andthe balance of 92 to 98% comprising a vinylidene chloride copolymer.Although this film has proven commercially useful in many packagingapplications requiring high oxygen barrier, it does not address theneeds of packaging applications such as gas (CO₂) producing cheeses, andproduce (vegetables) where a moderate O₂ barrier combined withrelatively high CO₂ transmission is often required.

A commercial composition currently used to make film for packaginggassing cheese includes 100 parts VDC/VC plus three liquid plasticizers:five parts of Plastolein 9759-A (a polymeric condensation product ofazelaic acid and 1,3-butandiol), three parts of Plasthall HA7A, apolymeric plasticizer of polyester of adipic acid and propylene glycol,and one part Plas-chek 775, an epoxidized soybean oil. It has been foundthat blends with liquid plasticizers do not free flow very well and thusrequire a grinding step before extrusion. This process, coupled withsome migration of the liquid additives out of the VDC/VC matrix, causesinconsistency, i.e. lack of homogeneity in the composition of theresulting blend. This inconsistency in turn leads to inconsistency inthe O₂ and CO₂ transmission rates of packaging films made from theblend. This is important, because consistent O₂ and CO₂ transmissionrates are needed to insure a commercially acceptable package system.When these gas rates can be controlled predictably, the amount of theadditives can be adjusted to optimize the transmission rates for theintended end use.

Some cheese products are produced in such a way that the final cheeseproduct emits a significant amount of carbon dioxide over time. In suchcases, it is often desirable to provide a packaging material which ischaracterized by a relatively low oxygen transmission rate to protectthe cheese product from oxidative degradation in quality or shelf life.It is also desirable to have a sufficiently high carbon dioxidetransmission to allow the naturally generated CO₂ to escape from thepackage, and avoid undesirable ballooning of the package.

The present invention provides a composition, and film, with a highertotal amount of plasticizer and stabilizer than that shown in the art,and therefore a film suitable for packaging applications where O₂ andCO₂ transmission rates are Important. In preferred embodiments, theinvention provides not only a higher amount of plasticizer andstabilizer, but also a relatively lower amount of liquid plasticizer inthe blend. The resulting improvements in free flowability save an extragrinding step in preparing the composition, and also offer a morecontrolled, consistent O₂ and CO₂ transmission.

The present invention also provides good thermal stability for saranformulations, and they do not degrade to any significant extent duringextrusion. Methods for determining stability are well known, e.g. asdescribed in U.S. Pat. No. 5,202,188 incorporated herein by reference.

SUMMARY OF THE INVENTION

In one aspect of the invention, an extrudable vinylidene chloridecomposition comprises 100 parts by weight of at least one vinylidenechloride copolymer; between 4 and 15 parts by weight of a plasticizer;and between 4 and 15 parts by weight of an acrylate/styrene copolymer;wherein the total amount of plasticizer and acrylate/styrene copolymercomprises at least 9 parts by weight.

In another aspect, a polymeric film comprises at least one layercomprising 100 parts by weight of at least one vinylidene chloridecopolymer; between 4 and 15 parts by weight of a plasticizer; andbetween 4 and 15 parts by weight of an acrylate/styrene copolymer;wherein the total amount of plasticizer and acrylate/styrene copolymercomprises at least 9 parts by weight.

DEFINITIONS

As used herein:

"Plasticizer" means a material incorporated in a film to increaseflexibility, workability, or extrudability of the film.

"Acrylate/styrene polymer" means an additive that has acrylate orstyrene moieties, or both, such as methyl methacrylate/butylacrylate/styrene terpolymer, methyl methacrylate/butyl acrylate/butylmethacrylate terpolymer, or poly (alpha-methyl styrene).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventive composition preferably comprises 100 parts byweight of at least one vinylidene chloride copolymer; between 5 and 15parts by weight of a plasticizer; and between 5 and 15 parts by weightof an acrylate/styrene copolymer.

More preferably, the present composition comprises 100 parts by weightof at least one vinylidene chloride copolymer; between 6 and 12 parts byweight of a plasticizer; and between 6 and 12 parts by weight of anacrylate/styrene copolymer.

Most preferably, the present composition comprises 100 parts by weightof at least one vinylidene chloride copolymer; between 8 and 10 parts byweight of a plasticizer; and between 8 and 10 parts by weight of anacrylate/styrene copolymer.

Preferably, the total amount of plasticizer plus acrylate/styrenecopolymer is at least 10 parts (to be added to 100 parts of thevinylidene chloride copolymer composition). More preferably, the totalamount of these additives is at least 12 parts, even more preferably atleast 14 parts. Most preferably, at least 16 parts of the additives areused.

Higher total levels of plasticizer and acrylate/styrene copolymer in thecomposition provide O₂ and CO₂ transmission that is very suitable forpackaging gassing cheese. In addition, as the total level of theseadditives goes up, the CO₂ transmission typically increasesproportionally more than the O₂ transmission. This can be seen in Table3, where the CO₂ /O₂ ratio shows an increase with an increased totalamount of plasticizer plus acrylate/styrene copolymer. Some of thisincrease is dependent on the specific materials used.

The advantage of this ratio increase is that the CO₂ transmission ratecan be beneficially increased (very useful in gassing cheeseapplications) while effecting a smaller increase in the O₂ transmissionrate. This latter value is desirably kept relatively low so thatoxidative degradation of the cheese or other product is retarded.

The composition can be made into monolayer or multilayer film, orientedor unoriented, by means well known in the art.

In a preferred embodiment, a film comprises a four layer film structure:sealant layer/core layer/barrier layer/abuse layer where:

sealant layer=90% ethylene vinyl acetate copolymer (EVA) (6%VA)+10%linear low density polyethylene (LLDPE) which is an ethylene/octenecopolymer (6.5% octene);

core layer=ethylene vinyl acetate copolymer (EVA) (15%VA);

barrier layer=(as defined in the Tables); and

abuse layer=92.5% ethylene vinyl acetate copolymer (EVA) (9% VA) and7.5% linear low density polyethylene (LLDPE) which is an ethylene/octenecopolymer (6.5% octene).

The sealant and abuse layers can comprise any suitable polymericmaterial, but preferably comprise an ethylene alpha olefin copolymer, anethylene unsaturated ester copolymer, an ethylene acid copolymer, otherpolyolefins, or any combination or blend thereof. The core layer, whichis optional, can also comprise any of these materials. Ethylene alphaolefin copolymers are preferably copolymers of ethylene with one or morecomonomers selected from C₃ to C₁₀ alpha olefins. Both heterogeneous andhomogeneous materials can be used. Examples of heterogeneous materialsinclude LLDPE (linear low density polyethylene), VLDPE (very low densitypolyethylene), ULDPE (ultra low density polyethylene), and the like.Homogeneous materials can include metallocene catalyzed polymers (MCP)such as those available from Exxon under the EXACT trademark, andavailable from Dow under the AFFINITY trademark.

Ethylene unsaturated ester copolymer refers to materials such asethylene vinyl acetate copolymer (EVA), ethylene alkyl acrylatecopolymers such as ethylene ethyl acrylate copolymer (EEA), ethylenemethyl acrylate copolymer (EMA), and ethylene n-butyl acrylate copolymer(EnBA). It also refers to ethylene methyl methacrylate copolymer (EMMA).

Ethylene acid copolymer refers to materials such as ethylene acrylicacid copolymer (EAA), and ethylene methacrylic acid copolymer (EMAA).Metal salt neutralized or partially neutralized versions of thesematerials, such as ionomer, are also included herein.

EXAMPLES

Table 1 identifies the compositions used in the examples.

Nine compositions of the present invention, as well as a controlcomposition, were tested for thermal stability (shown as degradation inminutes) and free flowability. The results are shown in Table 2.

Table 3 shows the O₂ and CO₂ transmission rates of four additionalexamples, compared with the same control. The control composition formspart of the B 625 bag commercially made by Cryovac Division of W.R.Grace & Co.-Conn. Oxygen transmission is measured by ASTM 3985; carbondioxide transmission is by a coulometric test method; both propertieswere measured using Mocon equipment.

                  TABLE 1                                                         ______________________________________                                        MATERIAL      TRADENAME   SOURCE                                              ______________________________________                                        VDC.sub.1     PV324       SOLVAY                                                VDC.sub.2 MA134 DOW                                                           P.sub.1 Plastolein 9759-A HENKEL                                              P.sub.2 Plasthall HA7A C. P. HALL                                             P.sub.3 Plas-Chek 775 FERRO                                                   P.sub.4 Citroflex A-4 MORFLEX                                                 S.sub.1 Metablen P700 ATOCHEM                                                 S.sub.2 Metablen L1000 ATOCHEM                                              ______________________________________                                    

where:

VDC₁ =vinylidene chloride/vinyl chloride copolymer;

VDC₂ =vinylidene chloride/methyl acrylate copolymer;

P₁ =polymeric condensation product of azelaic acid and 1,3-butandiol;

P₂ =polymeric plasticizer of polyester of adipic acid and propyleneglycol;

P₃ =epoxidized soybean oil;

P₄ =plasticizer of acetyl tri-n-butyl citrate;

S₁ =methyl methacrylate/butyl acrylate/styrene terpolymer; and

S₂ =methyl methacrylate/butyl acrylate/butyl methacrylate terpolymer.

In the tables, "phr" means pounds per hundred (weight units) ofmaterial. For example, in the control, 100 pounds of saran was blendedwith 5 pounds of P₁, 3 pounds of P₂, and 1 pound of P₃. An equivalent is"parts by weight".

                  TABLE 2                                                         ______________________________________                                                 COMPOSITION  DEGRADATION  FREE                                         EXAMPLE (phr) (minutes) FLOW                                                ______________________________________                                        1        100 VDC.sub.1  +                                                                           21.0         very poor                                     5 P.sub.1  +                                                                 (Control) 3 P.sub.2  +                                                         1 P.sub.3                                                                    2 100 VDC.sub.1  + 24.0 excellent                                              4 P.sub.3  +                                                                  5 S.sub.1                                                                    3 100 VDC.sub.1  + 21.0 excellent                                              4 P.sub.3  +                                                                  10 S.sub.1                                                                   4 100 VDC.sub.1  + 43.0 very good                                              8 P.sub.3  +                                                                  5 S.sub.1                                                                    5 100 VDC.sub.1  + 38.0 very good                                              8 P.sub.3  +                                                                  10 S.sub.1                                                                   6 100 VDC.sub.2  + 44.0 excellent                                              4 P.sub.3  +                                                                  5 S.sub.1                                                                    7 100 VDC.sub.2  + 68.0 excellent                                              4 P.sub.3  +                                                                  10 S.sub.1                                                                   8 100 VDC.sub.2  + 54.0 excellent                                              4 P.sub.3  +                                                                  10 S.sub.1                                                                   9 100 VDC.sub.2  + 70.0 very good                                              4 P.sub.3  +                                                                  10 S.sub.1                                                                   10  100 VDC.sub.2  + 70.0 very good                                            4 P.sub.3  +                                                                  10 S.sub.1                                                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                COMPOSITION O.sub.2  trans.                                                                         CO.sub.2  trans.                                                                      CO.sub.2 /O.sub.2                         EXAMPLE (phr) cc/day.m.sup.2 cc/day.m.sup.2 ratio                           ______________________________________                                        1       100 VDC.sub.1  +                                                                          41.0      248.0   6.05                                       5 P.sub.1  +                                                                 (Control) 3 P.sub.2  +                                                         1 P.sub.3                                                                    11 100 VDC.sub.1  + 38.0 245.0 6.45                                            8 P.sub.3  +                                                                  8 S.sub.2                                                                    12 100 VDC.sub.1  + 31.0 200.0 6.45                                            5 P.sub.3  +                                                                  10 S.sub.1                                                                   13 100 VDC.sub.1  + 40.0 248.0 6.2                                             6 P.sub.3  +                                                                  10 S.sub.1                                                                   14 100 VDC.sub.1  + 37.0 278.0 7.5                                             8 P.sub.4  +                                                                  10 S.sub.1                                                                 ______________________________________                                    

Those skilled in the art will understand that modifications to thepresent invention can be made after review of the disclosure. Suchmodifications are deemed to be within the scope of the invention asclaimed.

For example, although four layer embodiments are disclosed as examples,those in the art will readily understand that one or more layers can bemade into a film having the benefits described.

The examples were made by a tubular extrusion coating process, wellknown in the art. However, other processes, such as flat cast extrusionor coextrusion, lamination, extrusion coating, extrusion lamination andthe like can be employed. These alternative processes are well known inthe art.

The examples were made by irradiating a substrate (sealant and corelayer) prior to extrusion coating of the barrier and abuse layers, andprior to orientation. Alternatively, films of the invention can betotally crosslinked by conventional means such as electronic or chemicalcrosslinking; post-irradiated, i.e. irradiated after orientation; ormade without crosslinking, or without orientation.

Orientation is accomplished by conventional means such as blown bubbleor tenterframe. Orientation ratios can be at any suitable range orratio, including 1:1 to 6:1 in either or both of the machine andtransverse directions.

What is claimed is:
 1. A thermoplastic film comprising a barrier layer,said barrier layer comprising:a) 100 parts by weight of at least onevinylidene chloride copolymer; b) between 6 and 15 parts by weight of aplasticizer; and c) between 6 and 15 parts by weight of a polymericadditive consisting essentially of acrylic and styrenic moieties,saidlayer having a ratio of CO₂ transmission rate to O₂ transmission rate ofat least about 6.2.
 2. The film of claim 1 wherein said barrier layercomprises between 6 and 12 parts by weight of said plasticizer andbetween 6 and 12 parts by weight of said polymeric additive.
 3. The filmof claim 1 wherein said barrier layer comprises between 8 and 10 partsby weight of said plasticizer and between 8 and 10 parts by weight ofsaid polymeric additive.
 4. The film of claim 1 wherein said plasticizercomprises a member selected from the group consisting of a polyesterplasticizer, an epoxidized oil, and a citrate.
 5. A thermoplastic filmcomprising a barrier layer, said barrier layer comprising:a) 100 partsby weight of at least one vinylidene chloride copolymer; b) between 6and 15 parts by weight of a plasticizer selected from the groupconsisting of a polyester plasticizer, an epoxidized oil, and a citrate,wherein said polyester plasticizer comprises a member selected from thegroup consisting of a polymeric condensation product of azelaic acid and1,3-butanediol, and a polyester of adipic acid and propylene glycol; andc) between 6 and 15 parts by weight of a polymeric additive consistingessentially of acrylic and styrenic moieties,wherein the barrier layerhas a ratio of CO₂ transmission rate to O₂ transmission rate of at leastabout 6.2.
 6. The film of claim 4 wherein said epoxidized oil comprisesa member selected from the group consisting of epoxidized linseed oiland epoxidized soybean oil.
 7. The film of claim 4 wherein said citratecomprises acetyl tri-n-butyl citrate.
 8. The film of claim 1 wherein thetotal amount of elements b) and c) in said barrier layer is at least 14parts by weight.
 9. The film of claim 8 wherein the total amount ofelements of b) and c) in said barrier layer is at least 16 parts byweight.
 10. The film of claim 1 wherein said polymeric additive ispresent in said barrier layer in an amount of about 10 parts by weight.11. A thermoplastic film comprising a barrier layer, said barrier layercomprising:a) 100 parts by weight of at least one vinylidene chloridecopolymer; b) between 6 and 15 parts by weight of a plasticizer; and c)between 6 and 15 parts by weight of a polymeric additive consistingessentially of acrylic and styrenic moieties, wherein the polymericadditive comprises a methyl acrylate/styrene copolymer;wherein thebarrier layer has a ratio of CO₂ transmission rate to O₂ transmissionrate of at least about 6.2.
 12. The film of claim 5 wherein said barrierlayer comprises between 8 and 10 parts by weight of said plasticizer andbetween 8 and 10 parts by weight of said polymeric additive.
 13. Thefilm of claim 5 wherein said epoxidized oil comprises a member selectedfrom the group consisting of epoxidized linseed oil and epoxidizedsoybean oil.
 14. The film of claim 5 wherein the total amount ofelements b) and c) in said barrier layer is at least 14 parts by weight.15. The film of claim 11 wherein said barrier layer comprises between 6and 12 parts by weight of said plasticizer and between 6 and 12 parts byweight of said polymeric additive.
 16. The film of claim 11 wherein saidplasticizer comprises a member selected from the group consisting of apolyester plasticizer, an epoxidized oil, and a citrate.
 17. The film ofclaim 11 wherein said polyester plasticizer comprises a member selectedfrom the group consisting of a polymeric condensation product of azelaicacid and 1,3-butanediol, and a polyester of adipic acid and propyleneglycol.
 18. The film of claim 11 wherein said epoxidized oil comprises amember selected from the group consisting of epoxidized linseed oil andepoxidized soybean oil.
 19. The film of claim 11 wherein said citratecomprises acetyl tri-n-butyl citrate.
 20. The film of claim 11 whereinthe total amount of elements of b) and c) in said barrier layer is atleast 16 parts by weight.